Abstract

The McMurray Formation comprises fluvial, estuary, delta and embayment deposits, for which the ichnology is well studied. Workers have recognized the dominantly brackish-water character of trace-fossil assemblages and those studies have contributed heavily to the interpretation of the deposit. This study considers the significance of lower McMurray Formation strata that contain a continental trace fossil assemblage.

The studied outcrops comprise a coarse-grained sandstone channel that overlies and crosscuts large-scale, unidirectionally-dipping, interbedded, finegrained sandstone and minor siltstone (i.e. Inclined Heterolithic Stratification referred to henceforth as IHS). Both lithofacies are overlain by a pedogenically altered clayey siltstone. The coarse-grained channel contains oriented wood clasts, and lacks bioturbation. The cross-cut IHS is devoid of large wood clasts and is commonly bioturbated. The IHS locally displays beds of granular sandstone that are lithologically similar to the channel sandstones.

The IHS-associated trace fossil assemblage is composed of a range of forms that hitherto now, have not been formally reported from the McMurray Formation. Trace fossils that contain irregular back-fill and burrow diameters, as well as varying orientations, are assigned to a range of adhesive meniscate backfilled burrows, including the ichnogenus Naktodemasis and Taenidium. Collectively, these trace fossils are associated with insect larvae and are taken as indicators of freshwater sedimentation. Other trace fossils, such as Siphonichnus and Cylindrichnus, are interpreted to be associated with low-salinity waters: as such the outcrops most likely represent sedimentation in the innermost estuary, with the coarse-grained channel representing a cross-cutting fluvial deposit.

The fresh-water and low-salinity trace fossil assemblage has not previously been described in the McMurray Formation. Importantly, the presence of continental and brackish-water trace fossils together offers a perspective of this McMurray locale. Lithologically, the fluvial and estuary deposits are clearly discernible.